This invention relates to improved photoreceptors utilizing flexible substrates and relatively brittle ionizable inorganic photoconductive material, the photoreceptor being obtained in accordance with a three-step ion bombardment cleaning, oxidation, ion-deposition process.
Photoreceptors, particularly those related to xerographic copying, traditionally comprise a photoconductive insulating layer such as an element or alloy thereof exemplified by selenium (amorphous or trigonal) and selenium alloys such as a Se--As, Se--Te, Se--Bi, etc., with varying amounts of a halogen. Such materials are customarily applied in charge blocking contact to a supporting metal- or metal-covered substrate. Suitable substrates for such purpose include, for instance, aluminum, steel, nickel, brass, NESA glass or corresponding metal-coated polymeric materials.
Photoreceptors comprising at least the above elements are generally given a uniform electrostatic charge and the sensitized surface then exposed to an image pattern defined by an electromagnetic radiation, such as light. Light impingement results in a selective dissipation of the initially applied charge leaving a positive electrostatic image. The electrostatic image is then customarily developed by applying oppositely charged marking particles onto the charge-bearing photoreceptor surface.
The above basic concept was originally described by Carlson in U.S. Pat. No. 2,297,691, and has been since amplified and redescribed in many related patents in the field. Generally speaking, photoconductive layers suitable for carrying out the above functions have a specific resistivity of about 10.sup.10 - 10.sup.13 ohm-cm, in the absence of illumination. In addition, their resistivity must drop at least several orders of magnitude where exposed to an activating radiation such as light.
Photoconductive layers meeting the above criteria also normally exhibit some loss in applied charge, even in the absence of light exposure. This phenomenon is known as "dark decay" and will vary somewhat with sensitivity and with usage of the photoreceptor. The existence of the problem of "dark decay" is well known and has been controlled to a substantial extent by incorporation of thin barrier layers such as a dielectric film between the base or substrate and the photoconductive insulating layer. U.S. Pat. No. 2,901,348 of Dessauer et al utilizes a film of aluminum oxide of about 25 to 200 angstrom or an insulating resin layer, such as a polystryene of about 0.1 to 2 microns thickness for such purpose. With some limitations, these barrier layers function to allow the photoconductive layer to support a charge of high field strength while minimizing "dark decay." When activated by illumination, however, the photoconductive layer and barrier layer must become sufficiently conductive to permit substantial dissipation of the applied charge in light-struck areas within a short period of time.
In addition to the above-indicated electrical requirements, it is also becoming increasingly important that photoreceptors meet rather stringent requirements with regard to mechanical properties such as flexibity and durability. Such additional criteria become particularly important in modern automatic copiers operating at high speeds where the photoreceptor is in the form of an endless flexible belt (ref. U.S. Pat. No. 2,691,450). While belt-type photoreceptors have many advantages, there are also serious technical problems inherent in their use. For example, high speed machine cycling conditions require particularly strong adhesion between the photoconductive layer and the underlying substrate. Unfortunately, however, some of the most sensitive and efficient photoconductive materials are relatively brittle as films and do not generally adhere well to flexing metal substrates. It is very important, however, that any interface between the electrically conductive supporting substrate and the photoconductive layer be stable and strongly adherent to both since changes at this point will have a substantial effect on the electrical properties of the photoreceptor.
The above problems have been considered and resolved to a substantial extent in a process described in a copending application having the same filing date by Lewis B. Leder, John C. Schottmiller and Harold H. Schroeder entitled "Improved Photoreceptor Fabrication" (U.S. Ser. No. 447,736, filed June 10, 1974 and now abandoned) wherein the substrate is initially bombarded by non-metallic ions under a DC glow discharge in the presence of air or an inert rare gas containing at least 1% by volume available oxygen. The initial step, as described, is followed or overlapped by further bombardment of the substrate with a mixture of high energy photoconductive cations, and non-metallic high energy cations such as nitrogen or argon plus uncharged vaporized photoconductive material. While the above-described process represents a substantial technical breakthrough in utilizing the more efficient brittle photoconductors in flexible belt-type photoreceptors, there still remains room for improvement. In particular, the production of high energy photoconductive cations in a glow discharge for bombardment purposes is relatively inefficient (up to 5% at best) and requires expensive electrical equipment of limited capacity. Moreover, the insulative nature of preferred inorganic photoconductive materials make it difficult to avoid the accumulation of some surface charges on the substrate in a DC glow discharge environment. In effect, this results in a substantially lowered efficiency in depositing ionic photoconductive material of like sign onto the substrate.
Neither the use of an oppositely charged screen nor an increase in field strength will completely avoid this problem.
It is an object of the present invention to obtain improved flexible durable photoreceptors suitable for high speed xerographic copying purposes.
It is a further object of the present invention to develop a new and more efficient method for utilizing brittle photoconductive elements in a class of high speed flexible photoreceptors without the need for complicated chemical pretreatment of the substrate.